Ultra wideband (UWB) technology uses base-band pulses of very short duration to spread the energy of transmitted signals very thinly from near zero to several GHz. When properly configured, UWB signals can coexist with other communication signals in the same spectrum with negligible mutual interference. The Federal Communications Commission (FCC) has specified UWB signal emission limits for UWB communication systems to prevent interference with other communication systems.
The emission profile of a UWB signal can be determined by examining its power spectral density (PSD). Characterization of the PSD of a “Time-Hopping Spread Spectrum” signaling scheme in the presence of random timing jitter using a stochastic approach is disclosed in an article by Moe et al. titled “On the Power Spectral Density of Digital Pulse Streams Generated by M-ary Cyclostationary Sequences in the Presence of Stationary Timing Jitter.” See IEEE Tran. on Comm., Vol. 46, no. 9, pp. 1135-1145, September 1998. According to this article, the power spectra of UWB signals consists of a continuous component and discrete components. When total power is the same, the discrete components present higher PSD than the continuous component.
Presently, multi-band orthogonal frequency division multiplexing (OFDM) is being considered for use with UWB communication systems. In multi-band UWB communication systems using OFDM, the UWB frequency band is divided into sub-bands and OFDM modulation is applied to each sub-band.
There is an ever present desire to increase the communication distances of communication systems such as multi-band UWB communication systems using OFDM. One way to increase communication distance is to increase the power used for transmissions. To increase transmission power while still conforming to the FCC emission limits for UWB signals, it is desirable to reduce the discrete components so that overall power can be increased while still conforming to the FCC emission limits. In traditional communication systems, scramblers are commonly used for timing recovery and equalization. Therefore, these scramblers may not be efficient and/or effective in reducing discrete PSD components in sub-bands of multi-band UWB communication systems using OFDM, e.g., due to the high pulse repetition frequency (PRF), i.e., about 100 Mbps to 500 Mbps, and the time division multiple access (TDMA) frame structure of these systems. Accordingly, improved methods and apparatus for reducing discrete PSD components in sub-bands of multi-band UWB signals using OFDM are needed. The present invention fulfills this need among others.